Baghdad ? The number of U.S. troops killed in Iraq edged closer to the milestone figure of 4,000 dead Saturday as a roadside bomb claimed the lives of three soldiers.

The bomb blew up the U.S. soldiers’ vehicle and killed two Iraqis in northwestern Baghdad, the military said in a news release. The military also reported that indirect fire, either a rocket or mortar, killed a U.S. soldier and wounded four others south of Baghdad on Friday.

The loss of life raised the number of U.S. troops killed since 2003 to 3,996, according to icasualties.org, which tracks American deaths. The bloodshed came three days after President Bush marked the anniversary of the U.S. invasion by declaring that the United States must win in Iraq. So far, 23 U.S. service members have died in March, making this month one of the quieter periods for U.S. casualties since the war began.

North of Baghdad on Saturday, a U.S. military helicopter opened fire near Samarra, killing six Iraqis who are believed to have worked for the Sons of Iraq, the American-funded neighborhood security groups that include many former Sunni insurgents.

An Iraqi army commander and one of the paramilitary group’s leaders both said the men were staffing a checkpoint when they came under fire. The U.S. military insisted that the men appeared to be engaged in suspicious activity at the site, where militants previously have planted roadside bombs. The U.S. Army said in a statement that it was not clear if the men belonged to the U.S.-backed units.

The Sons of Iraq movement, which began with tribes in western Anbar province rebelling against al-Qaida in Iraq, has led to a sea change as many former Sunni insurgents have decided to join forces with the U.S. military nationwide in battling Islamic radicals.

Maj. Majeed Abbas, one of the leaders of the Sunni fighters in Samarra, said he had alerted the Americans that his men were policing checkpoints in the region around Lake Tharthar, which is a popular route for insurgents moving between Anbar and Salahuddin province to the northeast.

“I contacted the (American) man in charge of the helicopters. … I told him to stop the helicopters and that these were our forces and that they were on duty,” Abbas said. He said that U.S. officers later asked him to pick up the bodies.

The Iraqi army commander in the area, Lt. Col. Dhia Mahmoud Ahmed, said he told the Americans that the Iraqi military had been aware of the checkpoint, but the U.S. officers said they had not been alerted to its presence. “God have mercy on the martyrs’ souls,” Ahmed said.

It was the latest in a string of controversial “friendly fire” incidents in which U.S. forces have called in airstrikes on its new Sunni allies. There are more than 80,000 Sons of Iraq members across Iraq. The groups have chafed at the slow pace of the Iraqi government to hire them to join the police and army.

In other violence, two people were killed in roadside bombings in Baghdad and the northern city of Kirkuk.

Staff writer Parker contributed from Baghdad and special correspondent Rasheed from Samarra. Staff writers Said Rifai, Raheem Salman and Saif Rasheed contributed to this report.